The immune system must eliminate cells of the body that have become diseased as the result of intracellular infection or oncogenic transformation. CD8+ cytotoxic T cells are responsible for completing this task and must be able to distinguish healthy cells from diseased ones. Major Histocompatibility Complex Class I (MHC class I) molecules are present on most nucleated cells and are responsible for presenting antigen at the cell surface for CD8+ T cell inspection. In order to present antigen, the MHC class I pathway relies on binding to short peptides, created from degraded cellular proteins. When the source protein is associated with a disease (such as a viral protein or tumor associated protein) specific CD8+ T cells will recognize the peptide- MHC class I complex and kill the cell. Because the peptide provides the ultimate specificity in this reaction, we are interested in understanding how these peptides are created. Our data indicates that proteins which are rapidly degraded following their synthesis (termed Defective Ribosomal Proteins or DRiPs) are responsible for efficiently generating a supply of peptides. This proposal seeks to determine which cellular metabolic pathways are used to direct the rapid degradation of DRiPs and to determine if DRiPs are necessary in physiologically relevant settings. We are focusing on the ubiquitin conjugation pathway, as ubiquitin coupling is intimately associated with protein degradation. The Specific Aims of this proposal are to determine which E3 ubiquitin ligases are necessary for DRiP antigen presentation, understand why conjugation of the ubiquitin-like molecule Nedd8 is necessary for DRiP antigen presentation, and identify and characterize chemical inhibitors of DRiP antigen presentation that target de-ubiquitinating enzymes (DUBs).